The present invention relates generally to touch screen controllers, and more particularly to improved pen-touch detection circuitry and methods therein which consume less power than the prior art in-touch detection circuitry.
The prior art is believed to include commonly assigned U.S. Pat. No. 6,246,394 entitled “Touch Screen Measurement Circuit and Method” issued Jun. 12, 2001 to Kalthoff et al., incorporated herein by reference. Also, commonly assigned U.S. Pat. No. 6,738,048 entitled “Touch Screen Controller” issued May 18, 2004 to Bernd M. Rundel, also incorporated herein by reference, is indicative of the state of the art.
As shown in “Prior Art” FIG. 1 herein, the '394 patent discloses a touch screen digitizing system which includes a touch screen unit or assembly 30,31 including a first resistive screen 30 with opposed x+ and x− terminals, a second resistive screen 31 with opposed y+ and y− terminals, and an ADC 22. The various terminals of touch screen assembly 30, 31 are connected to corresponding terminals of a touch screen controller (TSC) chip 1A including a first switch 19 which is coupled between a first reference voltage (ground) and the x− terminal, and a second switch 18 which is coupled between the x+ terminal and a second reference voltage +VCC for energizing the first resistive screen 30. A third switch 21 is coupled between ground and the y− terminal, and a fourth switch 20 is coupled between the y+ terminal and VCC for energizing the second resistive screen 31. Switching circuitry 15, 17 couples an input of the ADC 22 to the y+ terminal while the first resistive screen 30 is energized and the second resistive screen 31 is not energized, and also couples the input to the x+ terminal while the second resistive screen 31 is energized and the first resistive screen 30 is not energized.
More specifically, the various terminals of the resistive screens 30 and 31 are connected to the drains of the various corresponding driver transistors 18, 19, 20 and 21. The structure provides continuous calibration of the full-scale analog touch screen output of the full-scale digital output of the ADC 22. Control circuit 41 controls the various driver transistors, switches, and ADC 22 by means of conductors 42, and also includes a data register 48 which receives/updates the analog to digital conversion results from ADC 22 so they are available to be read by the host processor 3 by a control/data bus 40. Control circuit 41 also can generate a processor interrupt request signal IRQ on conductor 47. Touch screen controller 1A of FIG. 1 also includes pen-touch detection circuit 2A shown in FIG. 2.
FIG. 2 shows a simplified diagram prior art pen-touch detection circuit 2A which is included in touch screen controller 1A of FIG. 1. Touch screen assembly 30, 31 in FIG. 2 includes the x resistive sheet 30 and the y resistive sheet 31 as shown in FIG. 1. Pen-touch detection circuit 2A includes N-channel driver transistor 21 of FIG. 1 coupled between y−conductor 25 and ground, with its gate connected by conductor 46 to control unit 41.
x+ conductor 27 is coupled to the source of N-channel transistor 32, the drain of which is connected by conductor 36 to one terminal of a pull-up resistor 39, to the input of a buffer circuit 38, and to the drain of a N-channel transistor 35 having its source connected to ground. The other terminal of pull-up resistor 39 is connected to VDD. The gate of transistor 32 is connected to the output of an inverter 33 having its input connected by conductor 34 to control unit 41A. The gate of transistor 35 is connected by conductor 45 to control unit 41. The output of buffer circuit 38 produces a pen-touch signal PENTOUCH on conductor 37 and applies it to an input of control unit 41A. Control unit 41 produces an interrupt request signal IRQ on conductor 47 and applies it to an interrupt request input of host processor 3.
Prior art touch detection circuitry 2A in FIG. 2 indicates whether or not there presently is a touch point Q at which there is electrical contact between the x resistive sheet 30 and the y resistive sheet 31 (FIG. 1) due to pen-touch pressure at on touch screen assembly 30,31 at the point Q. When transistor 21 is turned on, pen touch detection circuit 2A is ready for pen-touch detection operation. There is a current path from VDD to ground, through pullup resistor 39, through transistor 32, x+ port conductor 27, the x and y touch screen resistances connected by the electrical contact effectuated by the touch point Q, and driver transistor 21. The signal IRQ generated by touch screen controller 1A (FIG. 1) on conductor 47 can represent a interrupt request for a “convert” command from host processor 3.
Transistor 35 in FIG. 2 is turned on in response to a signal from control unit 41 via conductor 45 in order to “hold” or maintain the pen-touch detection signal PENTOUCH at a low level after a touch point Q has been detected, because as soon as that happens, transistor 32 must be turned off to isolate the touch point coordinate conversion circuitry from the pen-touch detection circuitry so that control unit 41 can turn on the various driver transistors 18, 19, 20 and 21 shown in FIG. 1 to initiate conversion of the x and y coordinate voltages of the detected touch point Q into corresponding digital values by means of ADC 22.
In prior art pen-touch detection systems, the touch screen controller 1A must monitor the pen-touch detection status either asynchronously or synchronously. Asynchronous monitoring may cause internal glitches in the pen-touch detection circuitry, and synchronous monitoring needs to start the internal clock in advance of the pen-touch detection and requires a continuous-running clock. which results in increased power consumption.
In the prior art, initiation of the pen-touch detection operation is controlled by the host processor 3, which necessitates a continuously running internal clock system and also necessitates associated software required in the host processor 3 for relatively continual communication between the host processor 3 and the touch screen controller. Also, in the prior art the touch screen controller interrupts the host processor 3 to request a convert command (which causes the touch screen controller 1A to initiate analog to digital conversion of the analog coordinate voltages of the touch point Q). When host processor 3 is ready, it is issues the requested convert command. Host processor 3 performs the function of determining the pen-touch status and determining if the detected touch point Q is valid also is removed from the host processor 3. This requires a substantial amount of communication between the host processor 3 and the touch screen controller, which in turn increases overall touch screen system power consumption. If higher clock speeds are needed to increase the speed of communication between host processor 3 and touch screen controller 1A, overall system power is further increased.
Thus, there is an unmet need for a touch screen controller pen-touch detection system and method which results in substantially less system power consumption
There also is an unmet need for a touch screen controller pen-touch detection system and method which avoids the need for a host processor to continually poll the touch screen controller to determine pen-touch of a touch screen assembly coupled to the touch screen controller.
There also is an unmet need for a touch screen controller which relieves a host processor of the burden of determining whether a detected touch point is valid.